ATLANTA — Thermometers strained again on Saturday as the eastern half of the United States faced Day 10 of Heat wave 2012, with temps creeping up into the humid 100s as sweaty Americans sought out pools, ponds, oceans, and rivers for relief.

The stretch of brow-wiping weather has proved historic, shattering thousands of heat records since June, and leaving parts of the Washington, D.C., area sweltering without power and air conditioning after a rare “derecho” thunderstorm hit the area last weekend. Authorities say 23 people across the country have died from heat-related causes in what’s on pace to become the hottest year on record, leaping ahead of 1934.

Even Atlanta, affectionately known as “Hotlanta,” saw its highest recorded temperature last week. And those living in places like St. Louis, Mo., have found themselves at the epicenter of a quirky set of meteorological circumstances that, were one to prognosticate, are likely to make it a particularly long July, which under normal circumstances is already the hottest month of the year.

It’s the emerging seesaw pattern of intense heat separated by slightly cooler, sometimes storm-carrying weather that’s a major clue as to what’s going on with the weather, and why the rest of the summer is likely to play out largely along the same steamy lines.

Why is it so hot? Large swaths of the country are covered by a massive heat dome described by Wunderground.com meteorologist Jeff Masters as “one of the greatest in recorded history.” A heat dome is basically a lump of high-level, slow-moving high pressure air that blocks cooling winds, leaving mammals inside the dome sweltering in very hot temperatures, medium to high humidity, and relatively still air.

The domes are formed as air moves across the high plateaus of the desert Southwest, where they’re fed by baking temperatures, and then slide eastward on the jet stream. When trees, lakes, and rivers begin to predominate as the dome moves across the Mississippi River, moisture evaporates into humidity, helping to create smog in cities and driving up the “feel” of the heat. On Saturday, likely the peak of the current heat wave, that “feel” was expected to rise to a blistering 112 in Washington.

So why the giant heat dome all of a sudden?

Some have pointed to urban heat island effects and suspected warming from man-made greenhouse gases as playing a role in Heat wave 2012. But one certain culprit, experts say, is the same atmospheric effect that sparked a lot of the big snowstorms across the mid-Atlantic and Northeast in recent years: the North Atlantic Oscillation.

The massive oscillation helps set weather in motion, and its movement can have direct, long-term effects on specific regions. This year, the oscillation is moving particularly slowly, meaning that it’s not letting through the hot air coming off the North American shelf, causing that air, in essence, to back up across the country, pushing hotter temperatures northward.

Thankfully, many of those in the hot zone can expect some relief after Saturday, as a trough of “cold” air – at least 10 to 15 degrees cooler – is managing to push the dome along from the west.

Jeff Weber, a scientist with the University Corporation for Atmospheric Research in Boulder, Colo., told OurAmazingPlanet.com that the pattern is likely to keep repeating through the month, and perhaps the end of the summer, with about 10 days of uncomfortable heat at a time offset by shorter stretches of cooler weather. Rinse, dry, repeat.

With 56 percent of the US experiencing drought, the dynamics of that seesaw pattern – extreme heat followed by cooler air and possible rain – will continue to be closely watched, not just by cubicle workers daydreaming about the beach, but farmers across the nation’s breadbasket worrying about whether the rains will bring enough water to offset the dusty dog days.